Bearing cages for rolling-element bearings are generally comprised of two axially-spaced-apart side rings and a plurality of bridges connecting the side rings. The bridges are disposed one behind the other in the circumferential direction of the bearing cage and each pair of adjacent bridges forms a pocket for accommodating or retaining a rolling element. The pockets of the bearing cage respectively hold the rolling elements at defined intervals relative to each other and prevent direct contact between neighboring rolling elements, thereby reducing friction and thus heat generation in the bearing. The pockets also ensure a uniform distribution of the rolling elements around the entire circumference of the cage or rolling-element bearing and thus enable a uniform load distribution as well as a quiet and smooth running of the bearing.
Bearing cages are heavily stressed during operation by frictional, strain, and inertial forces. In addition, under certain circumstances, detrimental chemical effects (degradation or corrosion) can be caused by certain additives and substances. Design and material selection are therefore of critical importance for the operational reliability of the cage as well as for the operational efficiency of the bearing.
Rolling-element bearing cages typically comprise pressed cages or solid cages. Pressed cages for rolling-element bearings are usually manufactured from sheet steel, in some cases also from sheet brass. Solid cages for rolling-element bearings can be manufactured for example from brass, steel, aluminum, polymers, or phenolic resin.
Solid polymer cages are often manufactured using an injection molding process and are characterized by a favorable combination of strength and elasticity. Good sliding properties of polymer on lubricated steel surfaces and the smoothness of the cage surfaces in contact with the rolling elements lead to low cage friction, a correspondingly low heat generation in the bearing and barely measurable wear. The forces from the inertia of the cage also remain small due to the low material density. The excellent running properties of polymer cages even under lubricant starvation conditions permit continued operation of the bearing for some time without risk of seizure and secondary damage.
For example, polyamide 66, polyamide 46, polyetheretherketone (PEEK), phenolic resin or other polymer materials can be used as polymers for conventional injection-molded bearing cages.
With pure polymer cages, however, it is very difficult to manufacture cages for medium to large bearing diameters, for example a diameter of greater than approximately 300 mm, with the required quality. This is due, inter alia, to the thermal expansion coefficient of the polymer, which is substantially greater than steel. As a result, when heat is generated during operation, a clamping effect of the rolling elements in a polymer cage can result due to the expansion of the polymer. Further, due to the higher thermal expansion coefficient, shoulder guidance of a polymer cage cannot be ensured and the dimensional stability in relation to the bearing cage diameter is also worsened. Moreover, the strength of the polymer is significantly limited in the radial direction as compared to metal. Finally, complex and large injection molding tools/machines also are required to manufacture polymer cages having such large diameters, which in turn leads to unacceptably high manufacturing costs.